The performance of the axial system in the machine tool and the life span of the bearing have strong reliance on preload applied to bearings, in proper intensity regarding to rev count and utilizing condition, by which can reduce a vibration of the spindle. A preload applied on the spindle has an effect on: improving surface profile of a structure; increasing the life span and reducing heat of the spindle. Therefore selection and utilization of a preload according to the processing condition of the machine tool are necessary.
The condition and appropriate preload application of a machine tool includes: strong cutting work with low speed spindle rotation, which requires strong preload; light cutting work with high speed spindle rotation, which requires light preload. By that, applying the same 20 intensity of preload among those cases is not appropriate.
Basically, a bearing combination composed to supply proper preload on spindle for securing proper spindle hardness matching the working condition accordingly. However, for above reason, the preload should be selectively applied following the working condition of two: fast speed rotation; low speed rotation.
I admit there are lots of advantages lying on spindle comprising preload varying structure, while such structure comes with problems: requiring too much for installation; comprising complex structure; high cost problem, which leads to the restriction of actual use.
For reference, high speed spindle (1 million or plus rate of DN degree) experiences undesirable preload variation when the ball forced to move toward outer wheel of the bearing as pressure increases dramatically by centrifugal force derived from inner wheel and the ball of the bearing. In case of static preload method, means for applying static and variable preload is appropriate.
The purpose of applying preload to an axial system of a machine tool is to let spindle to select its position choice in the direction of spindle (axial direction) and radial direction, and to gain effect of: reducing vibration of spindle; hardening the intensity of the bearing; reducing resonance and vibration of spindle direction; reducing circling slip, revolve slip and rotary slip; maintaining proper position regarding orbit wheel. Thus, preload of the bearing have a huge effect on the intensity of the spindle, vibration, noise, heat and accuracy of work according to its method and preload condition. Mainly, static preload is for high rate rotation and static position preload is for low rate rotation and low variation of rotation. It is typical to apply high preload to high rate rotation and low preload to low rate rotation.
KR patent 0246309 introduces pre-pressure change device of main shaft bearing. The invention comprises: A cylinder fixed in either side to pressing member with minute gap and to the retention assembly, providing a piston at its inner side so as to apply pressure to pressing member for intensifying preload of the bearing when the main axial rotate in low rate; A bearing preload spring provided at the side of the retention assembly, pushing pressing member to apply preload on bearing if the main axial rotate in high rate. An oil supply pipe provided in the retention assembly to supply pressure to the piston when the main axial rotate in low rate; A elasticity spring continuously pushing the piston to its original position when the main axial rotate in high rate.
KR patent 96-29232 shows preload controlling device utilizing PZT (piezoelectric element), and KR patent 94-27675, 91-14403 and EP 00997657 and U.S. Pat. No. 4,611,934 show Bearing preload variation system.
The means of utilizing PZT for controlling preload stated in current method show fast reactance rate but require great deal of space and cost, restricting the actual application of the method.
KR patent 0925919 states bearing variable preload system utilizing compressed air comprising: A sleeve provided to apply preload to outer wheel of the bearing coupled to outer side of high speed rotatable spindle; The sleeve moves along with the piston forced by compressed air.
The overall Bearing variable preload systems have following problems.
1: Their structure are difficult to apply accurate preload with its structure providing the bearing sleeve installed between the outer wheel of the bearing and the housing, since the action point of outer wheel of the bearing moves to outer side.
2: Because of the structure of forming the bearing sleeve between the outer wheel of the bearing and the housing, the main axial goes relatively bigger or the bearing supporting the spindle becomes small. Minimizing the bearing increases the Maximum permissible RPM, while reducing the preload applied and in point of the size of preload, among the difference between the upper and the underside, the smaller shows shortening its life-span.